Joe BoudreauUniversity of Pittsburgh

The Meaning of Recent Experiments on Matter and Antimatter

Starr FoundationHilary Term 2003Lady Margaret Hall

A primer on matter:

A primer on antimatter:

* Not just lying around.

* Difficult to produce: it requires an accelerator, or other source of high-energy particles such as cosmic rays.

* Even more difficult to contain, because matter + antimatter can annihilate: the two particles are destroyed, their

mass turns into radiation.

* Antimatter has the same mass, opposite charge, as matter.

e+

e-

Matter (elementary)

electron quark

Matter (composite)

proton neutron nuclei.

atoms, e.g. hydrogen, oxygen

Antimatter (elementary)

antielectron = “positron” antiquark

Antimatter (composite)

antiproton antineutron antinuclei.

antiatoms, e.g. antihydrogen, antioxygen

Where can we observe/experiment on antimatter?

Paul Adrien Maurice Dirac(Nobel Prize 1933, for thetheory of the spinning electron)

Carl Anderson (Nobel Prizein 1936 for the discovery ofthe positron).

Owen Chamberlain (Nobel Prizein 1959 for the discovery of theantiproton).

Matter has the property of sticking together. The force binding an electronto the proton is called the electromagnetic force.

221

r

qqF

Matter/antimatter symmetry:

What is the force that binds an antihydrogen together?

Suppose we swap proton antiproton, electron positron.

221

221 ))((

r

qq

r

qqF

All of the atomic physics of antihydrogen is the same as that of hydrogen.

Since chemistry comes from atomic physics, all of the chemistry of antihydrogen is the same as that of hydrogen.

Atomic physics is governed by the electromagnetic force of James Clerk Maxwell, .. Its invariant to the operation we just described.

This operation is referred to as charge-conjugation symmetry , or “C”

Q: Does this only work for hydrogen atoms?

A: No, it generalizes in two important ways.

Not just forces, but also processes, e.g. H* -> H +

“Interaction”, a term to describe both forces, and processes.

The remarks about matter antimatter symmetry also applies to processes

Now to describe one class of recent (1980s) experiments on antiprotons.

None of the experiments in this class have shown any difference between matter and antimatter.

If they did, it would be a major discovery….

Prior to the 1980s, slow antiprotons were not available, only very fast beams.

The Bevatron used a proton beam on a copper target. Antiprotons are produced and separated from other debris.

The situation changed in the 1980’s:

LEAR: a machine to DECELLERATE antiprotons.The “Penning Trap”, a device to catch and hold them.

Many measurements have followed. What did we learn from these?

•The mass of the proton and antiproton are equal within the experimental precision of 1 part in 10 billion.

•The antiproton, like the proton, is as far as we can tell a stable particle. CERN ion trap experiments give lower limits of t > 3.4 months. Fermilab’s APEX experiment sets a limit of ~ 10 thousand years.

•The charge of the antiproton has been compared to that of the proton, and not found to differ.

•The magnetic moments have also been compared, and found not to differ.

•A new round of experiments with antihydrogen is now underway.

•Antihydrogen (antiproton + positron) has been produced:

• About 9 fast antihydrogen atoms in 1996 (PS210, CERN)• About 67 fast antihydrogen atoms in 1997 (E862, FNAL)• 50,000 cold antihydrogen atoms have been produced at CERN (Athena)

Antihydrogen decay in the ATHENA experiment (CERN)

The goal of this round of experiments is to check the atomic spectrum of antihydrogen and compare it to hydrogen.

Some of the long-lived spectral lines can be compared with a precision of a few parts in 1014, making this the most precise test yet of this kind of matter-antimatter symmetry.

Why? What would have to change in our understanding of nature?

* The description of our elementary particles in terms of field quanta.

* E.G. the photon (g) is the quantum of the electromagnetic field

* Electron and other elementary constituents, are quanta of of other fields.

* The theoretical basis for our understanding of particles and all of our calculational tools are based on this.

* The success of this model is its justification.

* The kind of symmetry which is in fact being tested here is actually something a little less than matter-antimatter symmetry.

* It’s a related symmetry called CPT

* It enforces equality between particle masses, lifetimes, spectra, charge, magnetic moment…

* And this symmetry is guaranteed by Wolfgang Pauli’s CPT theorem.

I started by explaining the electromagnetic force between proton andelectron, and its invariance to charge-conjugation.

I generalized this to the notion of an interaction, which includes notonly forces, but also processes. I promised to generalize it once again.

There is more to life than the electromagnetic force

Proton and electron also pull on each other with a gravitational force.

Charged particlesColoured Particles

The strong force

* keeps the proton together

The weak force

* everything in the table has “weak charges” painted onto it.* so everything feels this exceedingly weak force.* it is the only thing that a neutrino feels.* and causes the neutrino to scatter from electrons, nuclei.* but mostly the weak force is known for it decay process:

n p e

The second generalization we can make:

CPT symmetry applies to all of the known forces.

1956: Discovery of parity violation in the weak interaction.

Chien Shiung Wu

•The violation of parity was not expected.

• There was then a generally observed compensating violation of charge conjugation in the weak interaction:

- replacing left-spinning electrons with right-spinning positrons leaves the interactions invariant.

- this operation is called CP.

•Then CP fell, in 1964. But only in the weak interaction.

•And only CPT stands today (m=0, =0, q..)

• This perhaps explains why antimatter experiments look for CPT violation in antiprotons & antihydrogen.

•BUT: no deep physics principle was lost when C, P, CP violation were discovered, only aesthetics.

There is a obvious preponderance of matter over antimatter in the whole universe.

* If antimatter collided with matter elsewhere in the universe, it would produce anihilation radiation which we could detect.

We have not.

* No plausible mechanism has ever been detected which can separate matter, and antimatter, into separate regions.

* Cosmic rays detected on earth contain only about 0.01% antimatter:

-consistent with generation from cosmic ray collisions.

So where is the antimatter?

* Was the universe born asymmetric?

* Or did it develop an asymmetry?

Imagine a hot early universe with equal amounts of matter and antimatter.

Quarks, antiquarks, electrons, positrons, neutrinos, antineutrinos.

Each quark finds an antiquark, and annihilates with it.

Instead of all this stuff, we have only radiation.

More subtle mechanism depends on asymmetric production, rather than decay.

The marvelous thing: these mechanisms are being probed today in classic experiments.

About 1 part in 10 billion of the original soup remains... everything we now see in the universe!

Now suppose the antimatter simply decayed before anhihilation.

That solves the problem. But that is forbidden by CPT.

We need a very heavy particle, “Y” which can decay into both quarks,and into electrons:

Y

u

d

Y

e+

u

Y

u

d

Y

e-

u

V1

V1

V2

V2

W+

c

b

W-

c

b

The mechanism is very similar to the situation with W bosons, real particles known since 1983.

|Vbc|

|Vbc|

W+

c

b

But, that’s something that we think we know how to do

W+

c

b

W

u

d

Two processes can actually contribute to the decay.

In Q.M. the actually decay can go through both of these at the same time.

The strengths of the two reactions do not add like regular numbers. They add like sides of a triangle.

There is an angle (“phase”) that matters.

They can “collaborate”, or “compete”....

… giving constructive, or destructive interference.

Matter Antimatter

Recapitulation:

•The reason why an equal mix of W+ and W- can decay into an asymmetric mix of b quarks and antiquarks is that the two amplitudes for producing b or b(bar) interfere in a different way—because of angles that violates CP symmetry.

•This same scenario can be applied to the decay of even heavier, though still hypothetical particles, to explain the matter-antimatter asymmetry in the universe.

•If this is the case, the universe is the ultimate two-slit interference experiment:

b

d

d

b

B0

B0

B meson

Recent experiments now concentrate on these two systems:

anti-B meson

00 || BB

•Similar to light, which can also exist in two polarizations.•There is a phenomena in B mesons which is similar to the optical phenomena of Faraday rotation, or rotation of the polarization of light:

•Start with polarized light.•Decompose it into two states of circular polarization.•In a sugar solution these states propagate with different speeds.•So one helicity arrives early, or, there is a phase difference.•When the two beams recohere, the polarization has rotated.

In B mesons a similar thing happens but it is driven by the weakinteraction, not scotch tape or maple syrup:

b

d b

d

W W

t

t

B0 B0

B0

B0

B0

B0

Search for CP violation in the B system is a variation on this:

J/ K0s

•CP violation as a mechanism for the matter-antimatter asymmetry in the universe, remember, is not experimentally proven nor even accessible.

• The presumed mechanism for CP violation in the weak interactions is modelled, but not proven.

•The meaning of recent experiments on matter and antimatter: is that this model is beginning to be well-tested.

Wu

d

Wu

s

Wu

b

Wc

d

Wc

s

Wc

b

Wt

d

Wt

s

Wt

b

•So far all difference between matter and antimatter are due to the weak interaction in conjunction with the Higgs boson.

•There is a hint that similar mechanisms, at high energy, could have accounted for the matter antimatter asymmetry of the universe, but no evidence.

•Particle physicists continue to look for cracks in their “standard model” of physics for other sources of CP violation.